There are several methods known in the prior art for protecting metallic material surfaces against corrosive environmental influences. The coating of a metallic work piece that is to be protected with a coating of another metal is a commonly used and well established method of the prior art. In the corrosive medium, the coating metal can be electrochemically more or less noble than the material base metal. If the coating metal is less noble, it functions, in the corrosive medium, as a sacrificial anode relative to the base metal (cathodic corrosion protection). Although the protective function resulting from the formation of corrosion products of the coating metal is desired, the corrosion products of the coating often result in undesirable decorative and not in frequently functional deterioration of the work piece. In order to reduce or prevent, for as long as possible, the corrosion of the coating metal, so-called conversion layers are frequently used, especially on cathodically protecting less noble coating metals such as zinc or aluminum and their alloys. These conversion layers are reaction products of the less noble coating metal with the treatment solution, which reaction products are insoluble in aqueous media over a broad pH range. Examples for these so-called conversion layers are so-called phosphatizations and chomatizations.
In the case of phosphatizations, the layer which is to be protected is immersed into an acidic solution containing phosphate ions (see, for example, WO 00/47799). The acidic medium results in the partial dissolution of zinc from the coating. The Zn2+ cations thus released, together with the phosphate ions of the treatment solution form a poorly soluble zinc phosphate layer on the surface. Since zinc phosphate layers themselves only provide comparatively poor protection against corrosion, but an excellent adherent surface for varnishes and paints applied thereto, their main area of application lies in their function as base layers for the application of varnishes and paints.
In the case of chromatizations, the surface to be treated is immersed into an acidic solution-containing chromium(VI) ions (see EP 0 553 164 A1). In the case of, for example, a zinc surface, a part of the zinc dissolves. Under the reductive conditions which then prevail, chromium(VI) is reduced to chromium(III) which is precipitated in the surface film rendered more alkaline through the evolution of hydrogen inter alia as chromium(III) hydroxide or as poorly soluble μ-oxo- or μ-hydroxo-bridged chromium(III) complex. In parallel, poorly soluble zinc chromate(VI) is formed. As a result, there is formed a tightly closed conversion layer on the zinc surface which provides good protection against corrosive attack by electrolytes.
However, chromium(VI) compounds are acutely toxic and strongly carcinogenic so that a replacement for processes using these compounds has to be found.
As a replacement for chromatization processes using hexavalent chromium compounds, there have now been established a number of processes using various complexes of trivalent chromium compounds (see DE 196 38 176 A1). Since the protection against corrosion that may be achieved thereby is usually inferior to that achieved with processes using hexavalent chromium, an additional organic sealing layer is often applied to work piece surfaces, usually by deposition from aqueous polymer dispersions. Especially when using so-called black passivations, i.e. processes in which black layers are formed on zinc-containing surfaces by means of trivalent chromium compounds, the post treatment of this first conversion layer for improving the protection against corrosion is indispensible according to the prior art (see WO 02/07902 A2). A disadvantage of this additional process step using polymer dispersions is the formation of drain lines with work pieces coated on a rack and/or the sticking together of pieces coated in bulk. Furthermore, there are problems regarding the size accuracy of threads and the like resulting from the thickness of such organic sealings. If such sealings provide strong protection against corrosion, the adhesion to the coated surface is normally also very strong. This means that adhesion to parts of the coating apparatus is also very good, which renders their cleaning difficult. Moreover, any items with coating faults which are to be recycled through the entire coating process must be de-coated with considerable effort, which usually requires an additional process step.
Moreover, it is difficult to achieve friction coefficients μtot.>0.25 (DIN 946) for surfaces obtained by treatment with known polymer dispersions whose properties are determined mainly by the characteristics of the dispersed polymer.